1. Field of the Invention
The present invention relates to a display apparatus, more particularly to a liquid crystal display apparatus which is used as both a reflection type display and a transmission type display and a method of driving the same.
This invention also relates to a light source apparatus, having a scatter control member, which is preferable to a liquid crystal display apparatus.
2. Description of the Related Art
Since liquid crystal display (LCD) apparatuses can be designed flat and light, they are used as display apparatuses for various kinds of electronic apparatuses. As the idea of computerization is recently penetrating among people, portable personal computers (notebook type personal computers) and portable information terminals become popular. Because of the portability, such electronic apparatuses should suppress power consumption as low as possible. In this respect, portable electronic apparatuses use a reflection type LCD apparatus which uses no back-light to reduce required power accordingly. While such an LCD apparatus can obtain good contrast in the daylight with bright outside light (in a bright environment), it cannot provide a visible display in a dark place or in the night (in the dark). By contrast to the reflection type, a transmission type LCD apparatus with a back-light can give sufficient contrast in the dark with the back-light's luminance of about 20 cd/m.sup.2, but cannot give good contrast in a bright environment even if the back-light's luminance is 200 cd/m.sup.2. In view of this, an LCD apparatus which can be used as both a reflection type display and a transmission type display as shown in FIG. 37 has been developed. This LCD apparatus has an LCD panel 201, a semipermeable semireflection film (half mirror) 202 located at the back of an LCD panel 201, and a back-light system 203 positioned at the back of the semipermeable semireflection film 202.
The LCD panel 201 comprises electrodes 213 and 214 respectively provided on the opposing surfaces of substrates 211 and 212, aligning films 215 and 216, and polarization plates 218 and 219 provided on the outer surfaces of the substrates 211 and 212. The back-light system 203 comprises a lamp 204 of a straight tube or an L shape, and a light guiding plate 205 of a white acrylic plate provided in association with the LCD panel 201. The semipermeable semireflection film 202 reflects lights which is incident through the LCD panel 201 thereto.
Another LCD apparatus which has a back-light system 303 as shown in FIG. 38 is also developed. The back-light system 303 comprises a transparent electrode 305, a high-dielectric layer 306, an inorganic electroluminescence (EL) layer 307, a high-dielectric layer 308 and an electrode 309 sequentially formed on a glass substrate 304. The high-dielectric layer 308 is formed of barium titanate. The inorganic EL layer 307 consists of a binder and inorganic electroluminescence particles dispersed in the binder. Those layers 307 and 308 are both as thick as several tens of micrometers.
The inorganic EL layer 307 should be formed thick in order to prevent the generation of pinholes which would short-circuit the transparent electrode 305 and the electrode 309 and to emit a uniform light in emitting area and also to ensure sufficient light emission. The high-dielectric layer 308 should be so formed as to ensure uniform luminance control. Accordingly, the inorganic EL layer 307 and the high-dielectric layer 308 are thick enough to be opaque to the visible light. However, because of their thicknesses, the effective voltage to be applied to the electrodes 305, 309 should be set high.
The display functions of those LCD apparatuses will now be briefly described. In the diagrams, reference character X indicates outside light in a bright state, which reaches the LCD panel 201. This outside light X passes the LCD panel 201 and hits the semipermeable semireflection film 202. At this time, a part of the incident light X1 passes the semipermeable semireflection film 202 as transmitted light X3. The remaining component of the incident light X1 is reflected to be reflected light X2. This reflected light X2 reaches the LCD panel 201. Display light X4 according to the alignment stage of the liquid crystal goes out from the display surface to display an image. In the dark, the back-light system 203 or 303 is activated. Consequently, illumination light Y goes out from the light guiding plate 205 or the inorganic EL layer 307. This illumination light Y passes the semipermeable semireflection film 202 to become illumination light Y1. In this case, a part of the illumination light Y passes the semipermeable semireflection film 202, and this transmitted part of the illumination light Y becomes the illumination light Y1. As the illumination light Y1 comes to the LCD panel, display according to the alignment state of the liquid crystal becomes possible.
In these LCD apparatuses, however, the light guiding plate 205, the inorganic EL layer 307 and the high-dielectric layer 308, whose sizes match with the display area of the LCD panel 201, are opaque to the visible light and do not show reflectability so much. If those display apparatuses do not provide the semipermeable semireflection film 202 above the light guiding plate 205 or the inorganic EL layer 307, they cannot be used as a reflection type display.
In those LCD apparatuses, a part of the incident light X1 (the transmitted light X3) passes the semipermeable semireflection film 202. This undesirably reduces the amount of the reflected light X2 so that the good contrast cannot be achieved. The illumination light Y outgoing from the back-light system 203 or 303, when those LCD apparatuses are used in the dark, is partly absorbed by the semipermeable semireflection film 202 and transmits thereto, then it becomes the illumination light Y1. That is, the light from the back-light system 203 or 303 cannot be used efficiently for display. The provision of good contrast in the dark, therefore, requires an improvement on the luminescent performance. This increases the power consumption. Particularly, a portable LCD apparatus cannot avoid the problem of a shorter continuous display time. Further, because of the thick inorganic EL layer 307 and high-dielectric layer 308, a high applied voltage of approximately several tens of volts (V) is needed. It is thus necessary to enlarge the voltage generator in a portable display apparatus, which makes it difficult to achieve high-density implementation of the portable display apparatus.
The luminance of the outgoing light Y from the back-light system 203 varies according to the distance from the lamp 204. To emit planar uniform light, therefore, a separate scatter plate becomes necessary, which undesirably makes the apparatus thicker.
In an LCD apparatus which uses color filters to ensure multi-color display, the outside light or the light from the back-light, which comes to the color filters provided in the LCD panel 201 and are dispersed to predetermined wavelength ranges by the color filters before going out. That is, the red color filter absorbs lights of the other wavelength ranges than the red wavelength range and passes the light of the red wavelength range. But, the red color filter cannot completely cut off the lights of the other wavelength ranges than the red wavelength range so that those lights partially escape the filter, thus lowering the color purity. Further, actually, the red color filter absorbs the light of the red wavelength range slightly in addition to the lights of the other wavelength ranges than the red wavelength range. This LCD apparatus therefore suffers a low luminance and a smaller contrast ratio. In the case of a semipermeable semireflection type LCD apparatus provided with color filters on the display side with respect to the liquid crystal, in particular, the outside light should pass the color filters and the liquid crystal twice when the apparatus functions as a reflection type. As the transmission type, the light from the back-light passes the color filters and the liquid crystal once. By contrast to used as a transmission type, the ratio of the outgoing light to the incident light is significantly low when the display apparatuses are used as a reflection type. Further, there is a great difference in display luminance between the reflection type display and the transmission type display.
There is an ECB (Electrically Controlled Birefringence) type LCD apparatus as an LCD apparatus which displays colors without using color filters.
The reflection type ECB LCD apparatus is structured such that polarization plates having polarization axes are arranged on both outer surfaces of a liquid crystal cell, which has a liquid crystal sandwiched between a pair of substrates, and a reflection plate is provided under one of the polarization plates.
In such an LCD apparatus, the outside light incident to one of the polarization plates passes the polarization plate, and it becomes linearly polarized light. As the linearly polarized light passes the liquid crystal cell, it becomes elliptically polarized light due to the birefringence effect of the liquid crystal. Thereafter, as the elliptically polarized light passes the other polarization plate, it becomes linearly polarized light. The light outgoing from the other polarization plate, whose intensity varies wavelength by wavelength, becomes colored light according to the intensity of the light of each wavelength. This colored light is reflected by the reflection plate and travels along the opposite path to the aforementioned path and goes out from the polarization plate. Therefore, the reflection type ECB LCD apparatus achieves color display with the birefringence effect of the liquid crystal and the polarization effects of both polarization plates, without any color filters. Note that as the alignment state of a liquid crystal molecules varies in accordance with the level of the applied voltage, the birefringence of light changes. In other words, the polarization state of the light which goes out from the liquid crystal cell changes, in accordance with the level of the applied voltage. It is therefore possible to change the color of the same pixel by controlling the level of the voltage applied to the liquid crystal.
Even in such an ECB LCD apparatus, as shown in FIG. 39, the reflectance or the ratio of the outgoing light to the incident light considerably differs for each color or in accordance with the level of the voltage to be applied between the electrodes 203 and 214. This significantly deteriorates the display luminance balance.